In conventional supercalenders or multi-nip calenders, when the nips are closed, the set of rolls is supported from outside the zone of treatment of the web by means of forces which are substantially equal to what is called the pin load applied to the bearing housings of the rolls during running, or which forces are lower than the pin load. The pin load is commonly defined so that it includes the weight of all of the auxiliary equipment connected with the bearing housings of the roll, such as gap shields, doctors, and so-called take-out leading rolls, and also the weight of the portion placed outside the web width and the weight of the bearing system. This prior art has been described best in the paper by Rolf van Haag: “Der Weg zum Load Control-System”; Das Papier, 1990, Heft 7, in which the regulation of the linear load in a conventional supercalender is described. In such supercalenders, the rolls are positioned one above the other so that their middle portions are curved upwards or, in a very rare and special case, are fully straight. The intermediate rolls do not bend in the same way, as compared with one another. Owing to the mode of running, the nip loads in the set of calender rolls are such that the roll masses occurring in the area of the web to be calendered always act with full effect upon all the nip loads placed underneath the roll concerned. In such a mode of running, it is assumed that the set of rolls is curved in such a way during running that the rigidities of the rolls do not have a substantial effect on the uniformity of the linear loads, and attempts are made to operate the calender based on this assumption so that exclusively the linear loads of the upper roll and of the lower roll are regulated on the basis of measurements of quality.
In Finnish Patent No. 96,334, corresponding to U.S. Pat. No. 5,438,920 (incorporated by reference herein), a calendering method and a calender that applies the method are described, which calender comprises a variable-crown upper roll, a variable-crown lower roll and a number of intermediate rolls placed between the upper roll and the lower roll in nip contact with each other. The rolls are arranged as a substantially vertical stack of rolls on the frame of the calender.
A material web to be calendered is passed through the nips formed by the adjacent rolls. The nip load produced by the mass of the rolls in the stack of rolls is eliminated in a specific manner so that all the nips in the calender may be loaded with the desired load, which load is, in a preferred alternative embodiment, equally high in all nips. Thus, the calendering potential could be utilized substantially better than in the earlier calenders. In FI 96,334, it is one of the basic ideas of the prior art calender that rolls bending in the same way are employed in the calender. The conduct of such substantially equally bending rolls in the calender and the simple possibility, permitted by such rolls, of relieving the entire mass of the roll are described, in which case this prior art calender and calendering method differ essentially from the first-mentioned German prior art in the very respect that the effect of the masses of the rolls on the linear loads in the lower nips can be regulated freely.
The prior art described above involves an essential problem. If it is assumed that the natural deflections of the intermediate rolls in the calender without linear loads, i.e., when the nips are open, and the rigidities of the rolls as well as the masses are different, first it is to be stated that such rolls do not comply with those described in FI 96,334 or U.S. Pat. No. 5,438,920, in which all of the intermediate rolls had substantially equal deflections. In reality, the manufacture of such rolls, which substantially meet the absolute requirement stated in these publications without separate operations, is very difficult and also expensive, in which connection it has been ascertained that an entirely trivial algorithm of regulation of linear loads, which does not take into account minor differences between the rolls, is not adequate from the point of view of reliable operation of the calender.